Coated glass is manufactured for various purposes, the coating being selected to confer some particular desired property of the glass. Important examples of coatings for architectural and automotive glass are those designed to reduce the emissivity of the coated face in respect to infrared radiation (low-e coatings), coatings designed to reduce the total solar energy transmittance and coatings designed to provide a hydrophilic or self-cleaning glass surface. For photovoltaic applications glasses with transparent conductive oxide (TCO) coatings are very important. It is known that for example fluorine doped tin oxide (FTO) or aluminum doped zinc oxide coatings serve well for TCO and low-e coatings, titanium oxide coatings, especially with anatase crystal structure serve for self-cleaning coatings and iron-cobalt-chrome-based oxide coatings serve for near-infrared reflection coatings.
The coatings which are applied to glass should usually have a high and uniform optical quality. The coatings are usually applied to a thickness of between about 10 nm and 1500 nm, depending on the application. The coating material usually has a refractive index different from that of the glass material and thus variations in the thickness of a coating can give rise to objectionable interference effects, so uniform thickness is important for good optical quality.
Coatings on glass can be divided into two different groups, soft coatings and hard coatings. Soft coatings are typically applied by sputtering and their adhesion to the glass surface is rather poor. Hard coatings which typically have an outstanding adhesion and high abrasion resistance are typically applied by pyrolytic methods, such as chemical vapor deposition (CVD) and spray-pyrolysis.
In CVD the coating precursor material is in vapor phase and the vapor is caused to enter a coating chamber and flow as a well controlled and uniform current with the substrate being coated. The coating formation rate is rather slow and thus the process is typically carried out at temperatures exceeding 650° C., as the coating growth rates typically increases exponentially as the temperature is raised. The rather high temperature requirement makes CVD—process rather unsuitable for glass coating operations made outside the float glass process, i.e. for off-line coating applications.
In order to form thick coatings, typically coatings with thickness higher than 400 nm, at temperatures lower than approximately 650° C., it is conventional to use a spray coating apparatus for spraying a stream of droplets of coating precursor solution onto the substrate. The conventional spray pyrolysis system, however, suffers from a number of disadvantages such as the generation of steep thermal gradients and problems with the coating uniformity and quality. A great improvement to the process can be achieved by decreasing the size of the droplets as described in the applicant's currently non-public Finnish patent applications FI20071003 and FI20080217.
Another way of improving the spray pyrolysis process is to use electrostatic spray deposition. German patent publication DE 32 11 282 A1, Albers, August, 29.9.1983, describes a process for coating glass by particles or droplets where the particles or droplets have a different electrical potential than the glass. The temperature of the glass is between 400 and 900° C. The particle or droplet charging is carried out by an electrostatic spray gun conventionally used in coating conductive surfaces in a diverse range of applications like automotive finishes.
There are three primary ways in which liquids are charged in electrostatic spraying: direct conduction, corona, and induction. In direct conduction the spray material must have a relatively high conductivity and the voltage is applied to the source of the sprayed material. The spray is emitted from the spraying nozzle already charged and atomizes instantly. In a corona charging system the sprayed droplets are charged after atomization by passing through a corona field. This is an effective technique, but it poses safety hazards due to a high probability of arcs. Another drawback of this technique is that the liquid can land on the corona-producing electrode prior to charging causing a decrease in performance. Thus corona charging is usually used mainly with dry particles rather than with droplets. In induction charging the voltage is applied in proximity to the nozzle so that the liquid travels near the source and picks up some of the electric charge. To avoid current flowing back to the feed tank, the material has to have a high bulk resistivity. Due to its comparatively low charging efficiencies, mechanical assistance is required to atomize the spray, typically by rotary atomization, air assisted atomization, or a combination of both.
The voltages used in the electrostatic spraying are typically very high. The publication DE 32 11 282 A1 describes a voltage of 90 kV used in the spraying apparatus.
Electrostatic spraying would allow considerable improvement to homogeneous droplet deposition in spray pyrolysis. However, the practical problems of the current electrostatic spraying processes, mainly due to the high voltage, high process temperatures and soiling of undesired areas, have been so difficult that the electrostatic spraying process is not used in the production of pyrolytic glass coatings.
The international application WO 2004/094324 A1, Liekki Oy, Apr. 11, 2004, describes a method for charging particles, which particles are used for processing a material. According to the publication, an electrical charge is introduced in the particle formation with the gas reacting with the reactant at that stage of the process in which the gaseous reactant is oxidized and the particles are formed. Advantageously the electrical charge is introduced in the process with oxidizing gas that reacts with the reactant, such as oxygen. The charge is produced in the gas by means of a suitable charging method, preferably by a corona charger. The publication does not describe droplet charging.
In all spraying-based coating techniques it is preferable not to coat undesired parts of the object and the coating chamber. Especially when the coating process is based on small droplets, as described in the applicant's currently non-public Finnish patent applications FI20071003 and FI20080217, diffusion of the small droplets creates a tendency for the small droplets to penetrate also to undesired sections, which creates a need for shielding or cleaning these sections.
According to the prior art the droplets may also be electrically charged by electrically charging the gas used in a two-fluid atomizers such that the droplets become electrically charged. The electrically charged droplets are then deposited on the surface of the substrate using electrostatic forces by charging the substrate with opposite polarity in relation to the droplets. Accordingly the droplets and the substrate are charged with opposite polarity for enhancing the deposition of the droplets on the surface.
The problem with above mentioned prior art solutions is that when the substrate is charged for depositing the charged droplets on the substrate the deposition efficiency and the uniformity is substrate specific. This is due to the fact that each substrate material has different charging properties or electrical properties including ability to carry charge. Also the substrate thickness and volume may influence the deposition of the charged droplets and thus the formation of the coating on the substrate. Therefore the process and apparatus for coating a substrate using charged droplets does not work similarly to all substrates. This makes the coating difficult to control. Furthermore, the apparatus and process becomes complicated as each substrate has to be charged before the deposition of the charged droplets.